【Objective】 The posterior silkgland of silkworm is a very important organ where fibroin is synthesized and secreted. Since fibroin constructs silk-filament by amount of 75%-80%, the mechanism of fibroin secretion, cocoon quality and high yield can be elucidated by the study on the posterior silkgland. 【Method】 Using two-dimensional gel electrophoresis (2-DE) and image analysis system, the changes of protein composition in posterior silkgland cell of day 1 (D1) and D4 in the 5th instar stage from various breeds of silkworm Bombyx mori were investigated. 【Result】 Differences were observed between days and breeds and far less than those at the gene level through EST analysis. The change patterns of protein composition from D1 to D4 were diverse in various breeds. 【Conclusion】The results suggest that the secretion of fibroin is regulated by multi-protein. The site and protein for regulation vary with breeds, which leads to the difference of the capacity of fibroin secretion in the posterior silkgland cell between breeds.